The present application relates to a method for detecting the amplitude and/or phase position of an at least partly intensity-modulated signal, and a device for carrying out the method.
Phase- or delay-sensitive mixer device elements for distance measurement are known from the state of the art. Such elements are described for example in German patent applications DE 196 35 932 and DE 197 04 496 as so-called Photogate Photomixing Devices (PG-PMD) for electromagnetic radiation. As an alternative to the PG-PMD elements the mixer device elements can for example also be designed as MSM-PMD elements (MSM: metal-semiconductor-metal), such as are disclosed in WO 02/33922 A2.
A feature common to the mixer device elements known from the state of the art is that they have read-out electrodes between which a photoconductive material, in particular a semiconductor material, extends. Additionally two or more transparent modulation gates can be provided on the semiconductor material.
If the photoconductive part of the device element is lit by intensity-modulated electromagnetic radiation, preferably in the visible or infrared spectral range, the conductivity of the device element between the read-out electrodes changes, depending on the intensity impinging on the detector at the time. If amplitude-modulated current or voltage signals which are inverted relative to one another or phase-shifted by 180° are simultaneously applied to the modulation dates (in the case of PG-PMD elements) or also directly to the read-out electrodes (in the case of MSM-PMD elements), mixed signals are received at the read-out electrodes and modulated by the differential frequency between the frequency of the intensity modulation of the incident radiation and the reference frequency. The output signals at the read-out electrodes of the mixer elements also depend on the intensity of the radiation occurring. If the two output signals of the read-out electrodes are added together, a signal dependent on the intensity of the radiation occurring is obtained, containing merely the direct components (DC). On the other hand the differential signal between the two output signals of the read-out electrodes contains merely the correlated signal components. Therefore the differential signal also carries both the phase and also the amplitude information of the incident intensity-modulated radiation.
The described method is also called push-pull detection or “balanced detection”, as the modulation or read-out electrodes of the mixer element are connected by signals inverted relative to one another or phase-shifted by 180°. The push-pull method makes it possible to suppress the direct components and the asymmetries of the mixed structure intensity-modulated radiation by forming the difference between the two output signals of the mixer device, as described previously, and take account of only those signals which are correlated.
Since the amplitude and phase of the signal are unknown variables during measurements, at least two measurements with different phase positions of the reference signals are needed to measure the amplitude and phase of the measurement signal after the difference formation, depending on the modulation signal of the radiation impinging on the mixer and of the reference signal.
In the detectors known from the start of the art the two measurements are carried out either using a space-division multiplex method or using a time-division multiplex method.
In the space-division multiplex method a detector or picture element has two mixer devices. The two mixer device elements can be either arranged discretely one next to each other or also be an integrated component, e.g. form read-out electrodes arranged in crossover-style pairs. The decisive feature is that the reference signals of every detector element are push-pull-modulated while they are phase-shifted, ideally by 90°, relative to the reference signals of the respective second detector element. In this way two correlation functions of the signal to be measured can be measured and phase and amplitude evaluated. In the case of the space-division multiplex, approximately double the surface area of a single mixer element is needed for each picture element in order to be able to evaluate two autocorrelation functions simultaneously.
In the case of time-division multiplex the autocorrelation functions of the signal are measured in chronological succession, the reference signals of the mixer elements being phase-shifted relative to each other between two related measurements. The time-division multiplex method therefore requires twice as much measurement time as the space-division multiplex method. This is a disadvantage in particular for applications in which the measurement time plays a significant role. If for example distances in road traffic are to be detected, then short measurement times are essential if a driver or an autonomous system is to be given sufficient reaction time for example to avoid an obstruction.
On the other hand the object of the present invention is to provide a method and a device which make it possible to avoid the aforementioned disadvantages of the state of the art.